1. Technical Field
The present invention relates to an infrared photodetector, more particularly, the present invention relates to a structure with voltage-tunable and voltage-switchable photoresponses constructed of superlatuices and blocking barriers.
2. Description of Related Art
2-1. Semiconductor Infrared Photodetectors
Infrared photodetectors can be cataloged into two main groups. In one group, called thermal detectors, a change in some electrical property of the detector is induced by the temperature increment due to the absorption of the incident infrared radiation. In the other group, called photon or quantum detectors, the carriers in the material can be excited by photons with appropriate energy and become detectable by the external circuit. Although the photon detectors for the FIR detection need to be operated at cryogenic temperatures, they exhibit better detectivity and temporal response than the thermal detectors. For applications in which the sensitivity and response time are highly emphasized, photon detectors are the better choices.
The photon detectors can be further divided into several typesxe2x80x94intrinsic, extrinsic, internal photoemission and intersubband photodetectors. The extrinsic photodetectors need to be cooled to relatively low temperatures ( less than 40 K). The main disadvantage of the internal photon emission photodetectors is their low quantum efficiency because the carriers are activated by free carrier absorption. The other type of photon detectors that use bandgap absorption is classified as intrinsic photodetector. The material choices for intrinsic photodetectors are limited to those semiconductors with bandgap absorption in the FIR region. Among them, HgCdTe is the most important one. However, the growth of large area films of low bandgap materials with high uniformity still remains challenging. Therefore, wide bandgap based intersubband photodetectors with more mature growth and processing technologies are more suitable for the fabrication of large photodetector arrays.
Please refer to FIG. 1, which shows intersubband photodetectors that are made of multiple quantum wells or superlattices. For multiple quantum wells 10, adjacent wells 10 are separated by thick barriers 11. Because the coupling of electron wavefunctions between adjacent wells is negligible, electron energies in the quantum wells 10 are quantized into discrete levels. On the contrary, adjacent wells 10 in a superlattice 12 are separated by thin barriers. As a result of the coupling of electron wavefunctions between adjacent wells 10, minibands form in the superlattice region 12.
The superlattice 12 has broader spectral response than the quantum well 10 because of the transition between minibands instead of discrete states. In addition, the is coupling of electrons in adjacent wells 10 makes the superlattice 12 a low impedance structure. In this invention, the low impedance characteristic of superlattices 12 is utilized to design multicolor infrared photodetector.
2-2. Some State of the Art Intersubband Multicolor Infrared Photodetectors
Because imaging systems capable of multicolor detection are valuable in various applications including astronomical observation, military and medical science, the development of intersubband multicolor infrared photodetectors has drawn much attention in recent years. In the following, some state of the art multicolor infrared photodetector structures are recited.
Please refer to FIG. 2, which shows a Multi-stack infrared photodetector with separating conducting layers. As shown in FIG. 2, multistack infrared photodetector 2 is fabricated by stacking multiple quantum wells 21 designed for different wavelengths on the same substrate 21 during the growth of the structure. This kind of multicolor photodetector 2 can be further divided into two types (A. Kock, E. Gomick, G. Abstreiter, G. Bohm, M. Walther and G. Weimann, Appl. Phys. Lett 60, 2011, 1992; Sarath D. Gunapala, Sumith V. Bandara, A. Singh, John K. Liu, Sir B. Rafol, E. M. Luong, et al, IEEE Transactions on Electron Devices, 47, 970, 2000). (i) The different detector stacks are isolated by thick conducting layers. The operation of each stack is actually the same as if the stack is grown as a single photodetector 2. This structure simply integrates several different photodetectors 2 onto the same substrate 22. FIG. 2 shows a multicolor focal plane array realized with the multi-stack infrared photodetectors 2. Obviously, some extra process steps are needed to make only one detector stack 21 of a pixel to connect to the corresponding unit cell of the readout circuit. For short wavelength detection, the upper stacks designed for long wavelength are disabled by metal gratings 211; while for short wavelength detection, the bottom stacks are disabled by the lateral metal 212. The pixels shown in FIG. 2 for long and short wavelength interlaces spatially. Because pixels for different wavelengths occupy different physical locations, the chip area is utilized inefficiently. The maximum achievable resolution is therefore limited. In addition, extra process steps also increase cost and decrease yield rate.
(ii) Please refer to FIG. 3(b), which shows the structure of Multi-stack infrared photodetector with separating conducting layer. As shown in FIG. 3(b) (L. C. Lenchyshyn, H. C. Liu, M. Bunchanan and Z. R. Wasilewski, J. Appl. Phys. 79, 8091, 1996), two stacks 31, 33 for different wavelength regions are stacked on the same substrate 34 and are separated by a thick conducting layer 32. This kind of photodetector is operated as a two terminal device. The difference from the multi-stack photodetector mentioned in (i) is that the middle contact only serves as an internal connection for the two stacks 31, 33. The photodetector operates as if there is two discrete photodetectors connect in series.
Please refer to FIG. 3(a), which shows the small signal equivalent circuit. The measurable photocurrent generated in one of the stacks 31, 33 is determined by the dynamic resistance of stack 31 or 33. Because the current-voltage relation is not a linear, the dynamic resistance of the photodetector stack 31 or 33 changes with the operating point. As a result, by changing the total external bias, the dynamic resistance of stack 31 or 33 and therefore the portion of the generated photocurrent can be varied by the external bias. Although this structure can achieve multicolor detection, there are several disadvantages. First, the lack of accurate model for the current-voltage relation hinders the prediction in the design phase of the photoresponses versus bias voltage. Second, because the dynamic resistance of the short wavelength stack 31 is generally higher than the long wavelength stack 33 under low bias, short wavelength dominates the spectral response under low bias. In order to observe long wavelength photoresponses, the photodetector needs to be operated in a high bias voltage to saturate the dynamic resistance of the short wavelength stack 31. However, both dark current and noise increase under such high bias condition. Third, the dynamic resistance changes with the operating temperature and background radiation. Therefore the photoresponses are sensitive to the variation of the environment. Fourth, the long wavelength spectral response is unavoidable accompanied by strong photoresponses in the short wavelength region.
Please refer to FIG. 4, which shows a Multi-stack infrared photodetector without separating conducting layers. As shown in FIG. 4 (K. L. Tsai, K. H. Chang, C. P. Lee, K. F. Huang, J. S. Tsang and H. R. Chen, Appl. Phys. Lett. 62, 3504, 1993), this kind of photodetector also stacks photodetectors for different wavelengths. However, there are no conducting layers separating the different stacks 43. Multicolor detection in this structure is achieved by the electric field domains formed in the different stacks under different bias voltages. At low bias voltage, more of the applied voltage drops across the short wavelength stack to preserve current continuity. Therefore, the spectral photoresponse is dominated by the short wavelength stack under low bias conditions. To observe long wavelength responses, a high bias is needed for the long wavelength stack to acquire enough bias drop. This kind of multicolor photodetector also has the same disadvantages as the structure discussed in FIG. 2.
Please refer to FIG. 5, which shows a Compound multi-stack infrared photodetector. As shown in FIG. 5 (M. Z. Tidow, Xudong Jiang, Sheng S. Li and K. Bacher, Appl. Phys. Lett. 74, 1335, 1999), combine the structures discussed in FIG. 2 and FIG. 4, and photodetectors with more detection wavelengths can be achieved. However, this kind of photodetector suffers the same disadvantages discussed above.
Please refer to FIG. 6, which shows an Asymmetric quantum well infrared photodetector. As shown in FIG. 6 (E. Martinet, E, Rosencher, F. Luc, Ph. Bois, E. Costard and S. Delaitre, Appl. Phys. Lett. 61, 246, 1992), the use of asymmetry quantum well enhances the oscillator strength for the bound-to-continuous transition at the expense of the reduction of the oscillator strength for the bound-to-bond transition. Because electrons excited to the second bound state can not tunnel through the barrier under low biases, the photoresponses at low biases are dominated by the short wavelength due to bound-to-continuous transitions as shown in the inset of FIG. 6. Under a high bias, the excited bound state electrons can tunnel through the barrier and contribute to the long wavelength spectral response. Therefore multicolor detection is allowable in this structure. Since both the detecting wavelengths and the tunneling probability for the bound electrons are related to the structure of the asymmetry quantum well, it is impossible to control the detecting wavelengths and optimize the performances individually. The lack of the design flexibility is the main drawback of this type of multicolor detector.
Please refer to FIG. 7, which shows Coupled quantum wells infrared photodetector. As shown in FIG. 7 (M. Z. Tidrow, K. K. Choi, C. Y. Lee, W. H. Chang, F. J. Towner and J. S. Aheam, Appl. Phys. Lett. 64, 1268, 1994), several quantized states exist in the structure because of the coupled quantum wells 72, 73. Electron transitions between different states make multicolor detection possible. However, this structure also lacks of the design flexibility as the asymmetric quantum well structure.
Please refer to FIG. 8, which shows Stark-effect quantum wells infrared photodetector. Using the Stark-effect in asymmetry quantum wells, the detecting wavelength can be tuned continuously by the applied bias. The disadvantages of this kind of detector are the narrow tuning rage, the dependence of responsivity on the applied bias, and the lack of design flexibility.
In this invention, the superlattice-based structure is proposed to overcome the disadvantages of the aforementioned multicolor infrared photodetectors. Before introducing the embodiment of this idea using GaAs/AlxGa1-xAs system, the advantages of the superlattice is introduced.
In the following, the advantages of using superlattice in the design of infrared photodetectors are discussed.
(a) The superlattices have wider detection ranges than quantum wells due to the minibands. By designing suitable energy filter for the photoelectrons, voltage-tunable multicolor photodetector can be realized.
(b) Lower operating voltage and power consumption. This can save considerable power consumption for the operation of a focal plane array.
(c) By adjusting the parameters related to wells and barriers of the superlattice, the superlattice minibands can be flexibly tuned.
(d) The resistance of the superlattice is low. Since the voltage drop across the superlattice is negligible, the voltage distribution is determined by high impedance is blocking layers.
(e) The superlattice structure is simple and the total thickness is thin. Therefore, the growth time and cost can be reduced.
These characteristics make the superlattice suitable for the design of infrared photodetectors. For example, a voltage-tunable infrared photodetectors can be realized by a superlattice 81 with a high-impedance blocking barrier 82 as shown in FIG. 8. FIG. 9 shows the measured spectral responses. The blocking layer 82 of this photodetector design not only reduces dark current but also serves as an energy filter for the photoelectrons. Because the impedance of the blocking layer 82 is much higher than the superlattice 81, the external bias almost totally falls on the blocking barrier 82. Therefore, the effect of the blocking layer 82 as an energy filter can be tuned by changing the bias voltage. Under low biases photoelectrons excited by short wavelength light with energy higher than the blocking layer 82 can pass through the blocking layer 82 and contribute to photocurrent. However photoelectrons excited by long wavelength light do not have sufficient energy to pass through the blocking layer 82. As a result, the spectral responsivity is dominated by short wavelength at low biases. When external bias increases in magnitude, the photoelectrons excited by long wavelength light can tunnel through the blocking layer 82 by the assistance of the electric field. Therefore, at high biases, both long and short wavelength responsivity can be observed. The simple structure containing a superlattice 81 and a blocking layer 82 illustrates the use of superlattice for the design of a multicolor infrared photodetector. (Applied Physics Letters, 80, p. 2251, 2002). Another illustration of using superlattice to suppress darkcurrent and increases the operating temperatures can be found in the paper (Proc. SPIE Vol.4288, p. 151-162, California, USA. 2001). The two examples show the flexibility of using superlattice in the design of the infrared photodetectors.
The present invention further utilizes the characteristics of the superlattice to design a double-superlattice structure with a blocking barrier to separate the two superlattices designed for different wavelength regions. The photoresponses caused by the superlattices under both voltage polarities can be optimized individually by the design of the barrier heights at both sides of the blocking barrier. Besides, the operation voltage magnitude can be controlled through the thickness of the blocking barrier. This invention provides a very flexible method to realize multicolor infrared photodetectors and improves many of the disadvantages of the other multicolor photodetector design mentioned above. In the following, the embodiment of this invention is illustrated.
This invention provides an infrared photodetector structure with voltage-tunable and -switchable photoresponses constructed of superlattices and blocking barriers, wherein the superlattice-based structure is proposed to overcome the disadvantages of the aforementioned multicolor infrared photodetectors. Before introducing the embodiment of this idea using GaAs/AlxGa1-xAs system, the advantages of the superlattice is introduced.